1. Field of the Invention
This invention relates generally to devices for making a permanent record of electronic information and more specifically to thermal print heads.
2. Description of the Prior Art
With the advent of high speed data processing and delivery systems, it has become necessary to develop high speed, high accuracy and economical means for rendering the data or information into permanent, readable form. The data processing industry has attempted to accomplish these goals utilizing conventional typewriter technology; line printers derived from lithograph techonolgy; and new technologies including rotary impact print heads such as the devices known as "daisy wheels;" impact matrix printers; and the subject of this invention, thermal printers. The applications of each of these technologies differ regarding the speed of operation, the total volume of materials printed, the type of printing medium utilized, the number of moving parts and the total cost of manufacture.
Thermal printers, such as the present invention, are designed to minimize moving parts and cost of construction. Thermal printers are not particularly applicable to situations with a high volume of data printed because they require specially treated paper which is more expensive than ordinary paper. Thermal printers are particularly utilized in calculators, printing scientific instruments, computer terminal and other applications with low volume paper requirements.
A thermal printer operates by causing the application of a point source of heat to specially treated paper. The point of heat causes the special paper to darken at the point of application in a dot form. The typical manner in which characters are formed using thermal printers is by the use of a matrix, that is an N.times.M array of dots. A microprocessor or other data processing unit typically determines the specific members of the array which will be darkened so as to create the letter, numeral or figure desired.
A thermal printer consists of a microprocessor or other unit which translates incoming data into a printable form, a system to deliver signals from the microprocessor to the printing head, high-current drivers which can deliver the required amount of energy to burn the dots, a print head upon which the point heat sources are created and a system for bringing the paper in contact with the print head. Various prior art thermal printers utilize a movable print head capable of moving laterally across the paper much in the same manner as a typewriter carriage. Other applications of thermal printers utilize a fixed print head and move only the paper relative to the print head. In the applications where a fixed print head is used, the print head must extend the entire usable width of the paper to print a full line.
One common element of the prior art print heads known to the inventors herein, is that the electronic driver circuits for the burn points, commonly transistors, are located externally to the print head itself. Consequently, all of the logic and switching mechanisms are located external to the print head. A disadvantage of using structures wherein the logic circuitry is exterior to the print head is that the number of input points on the print head is increased such that it equals or is a substantial subset of the number of burn points.
An example of a prior art print head is described in a publication of Displaytek Corporation describing part number DC-1157, 5.times.7 dot matrix, thermal print head component. This print head utilizes individual input leads for each of the 35 burn points in its 5.times.7 matrix. Furthermore, the input section of the print head is considerably wider than the printing section where the burn points are located. This makes it difficult to array print heads of this variety next to one another so as to form a line. The Displaytek DC-1157 and other prior art printing heads such as those manufactured by Gulton Industries, and described in Application Bulletin No. 201, utilize matrices which print the entire matrix at one time rather than printing the individual rows non-simultaneously. Printers which print an entire character matrix at one time, and have a discrete spacing between such matrices are incapable of appropriately representing graphic figures or other characters for which they are not specifically programmed. If fixed matrix printers are programmed to represent a graphic image such as the arc of a curve, the speed of printing is slow.